Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a continuous sheet, and specifically relates to reduction of damage to a rotating member.
Description of Related Art
In an electro-photographic image forming apparatus, a laser beam is irradiated uniformly on a charged photoreceptor drum based on image information to form an electrostatic latent image on the surface of the photoreceptor drum. Toner is supplied to the photoreceptor drum on which the electrostatic latent image is formed, the electrostatic latent image is made visible and the toner image is formed. After the toner image is transferred on a sheet with a nipping portion formed between an intermediate transfer belt and a secondary transfer roller, the toner image is fixed to the sheet by applying heat and pressure with the fixing unit, and the image is formed on the sheet.
The following method of fixing with a fixing unit is known. A heating roller including a heating source, an upper pressure roller and a lower pressure roller are provided. A fixing belt is suspended from the heating roller and the upper pressure roller. The heat generated from the heating source is transmitted to the sheet through the heating roller and the fixing belt to heat the sheet, and pressure is applied to the sheet by forming a nipping portion between the fixing belt and the lower pressure roller by applying pressure with the upper pressure roller. The heat generated from the heating source is first transmitted to the fixing belt, and then transmitted to the lower pressure roller through the fixing belt. The sheet is heated with the heat of the fixing belt and the heat of the lower pressure roller. The fixing method using such fixing belt is advantageous in that the time necessary to raise the temperature to the temperature suitable for fixing (raising the temperature of the fixing member to a temperature so that the toner image can be fixed to the sheet) and the power consumed to raise the temperature to the temperature suitable for fixing is low compared to a fixing method which does not use a fixing belt (method in which the heating roller comes into contact with the sheet to heat and apply pressure to the sheet).
Continuous sheets, such as a rolled sheet or a folded continuous sheet, in which the sheet is longer than the length of the apparatus itself (in other words, longer than the sheet passing path provided from the sheet feeding unit to the sheet ejecting unit of the apparatus itself) may be used in an image forming apparatus. When the user inputs an instruction to stop image forming during the image forming, or the image forming stops due to error processing for some reason, the image forming is stopped after ejecting the sheet from the ejecting unit or conveying of the sheet is stopped upstream from the fixing unit in the sheet passing path in an image forming apparatus which uses a non-continuous sheet (for example, sheet with A4 size). However, such process is not possible in the image forming apparatus using the continuous sheet. Therefore, the conveying of the continuous sheet stops with the continuous sheet remaining in the fixing unit. If the continuous sheet is removed from the apparatus itself when the image forming is stopped, the continuous sheet does not remain in the fixing unit. However, it is troublesome to remove the continuous sheet from the apparatus main body and to insert the sheet again.
When the image forming is stopped as described above, and the continuous sheet is left held in the nipping portion formed by the fixing belt and the lower pressure roller, although the power is not supplied to the heating source and the temperature of the fixing unit is lowered when the image forming is stopped, the continuous sheet is damaged by the heat of the fixing unit, for example, the shape or the color changing. Therefore, when the conveying of the continuous sheet is stopped in a fixing method using heat such as a fixing belt, there is a method of separating the fixing belt from the lower pressure roller to release the nip to avoid contact of the continuous sheet to the fixing belt so that the continuous sheet does not come into contact with the fixing belt and the heat is not directly transmitted to the continuous sheet. Since the continuous sheet does not come into contact with the fixing belt, the heat of the fixing unit is not directly transmitted to the continuous sheet. However, since the heat is transmitted to the continuous sheet through the air around the fixing belt in the fixing unit, deforming, etc. of the continuous sheet occurs although the amount of deforming is small compared to when the continuous sheet is in contact with the fixing belt.
When the image forming is started again, since the supply of power to the heating source is stopped and the heat of the fixing unit becomes lower, the temperature is raised to the temperature suitable for fixing as preparation for image forming. In raising the temperature to the temperature suitable for image forming, when heat is provided by the heating source of the heating roller, the fixing belt is rotated in a state with the fixing belt separated in order to maintain the temperature of the entire fixing belt uniform. In this case, the continuous sheet may come into contact with the fixing belt even when the fixing belt is separated due to the deformed continuous sheet. If the fixing belt is rotated while the continuous sheet is in contact with the fixing belt, there may be problems such as scratches occurring in the circumferential direction of the fixing belt due to friction between the continuous sheet and the fixing belt.
The scratches on the fixing belt in the circumferential direction become a cause for lines in a formed image. Images with lines are not beautiful. Such images are not desirable to maintain the quality of the image forming apparatus.
The above example describes using a fixing belt as an example of the rotating member, but a similar problem occurs when the heating roller is used as the rotating member. In other words, if the continuous sheet is deformed when the heating roller is rotated separated from the lower pressure roller, there may be problems such as scratches occurring in the circumferential direction of the heating roller due to friction between the continuous sheet and the heating roller.
The above problem also occurs when the intermediate transfer belt is used as the rotating member. In other words, if the continuous sheet is deformed when the intermediate transfer belt is rotated separated from the secondary transfer roller, there may be problems such as scratches occurring in the circumferential direction of the intermediate transfer belt due to friction between the continuous sheet and the intermediate transfer belt. Since the temperature near the intermediate transfer belt is lower than the fixing unit, the influence of the heat to the deforming of the continuous sheet is small, but the continuous sheet may be deformed by absorbing moisture.
Japanese Patent Application Laid-Open Publication No. 2008-233770 describes stopping the continuous sheet at a certain time interval when the printing of the continuous sheet is stopped so that sagging of the continuous sheet (the above-described deform includes sag) does not occur.
However, Japanese Patent Application Laid-Open Publication No. 2008-233770 conveys and stops the continuous sheet at a certain time interval, and therefore, continuous operation is necessary. Moreover, the continuous sheet is conveyed at a certain time interval as a white sheet without forming images. Therefore, this wastes a large amount of continuous sheets, and extra continuous sheets become necessary. This is disadvantageous in terms of cost. Since the conveying and stopping at a certain time interval is not done while the power of the apparatus is turned off, the continuous sheet is deformed by heat and moisture when the power of the apparatus is turned off. Therefore, there is a possibility that the continuous sheet comes into contact with the rotating member when the power of the apparatus is turned on. In this case, if the rotating member is rotated with the continuous sheet in contact with the rotating member, there may be problems such as the scratches occurring in the circumferential direction of the rotating member.